1. Field of the Invention
This invention relates generally to a protective coating for electrical devices and in particular to a thin film assembly useful as a passivating coating for a semiconductor device.
2. Description of the Prior Art
In present day semiconductor technology, passivation layers are used to provide several functions for protection of the semiconductor device structure against environmental influences that arise in the manufacture and application of semiconductor devices. Passivation layers protect the semiconductor devices from the effects of moisture and contaminants which may occur during oxidation or other steps of the manufacturing process such as during assembly of circuits using the devices, or during the practical operation of the devices in circuit environments. By using passivation layers, production yield is increased and deleterious effects are minimized when the semiconductor device is operating in the field.
Especially desirable are passivation layers that effectively passivate surface states, which are electron energy levels at the surface of the semiconductor substrate, characterized by electrical charge and discharge having variable time constants. This phenomenon causes electrical drift, which may be short term or long term and which undesirably changes the characteristics of a field effect transistor. Surface states are "passivated" when a layer overlying the semiconductor surface interacts with atoms at the surface in such a way as to reduce the time constants characterizing the electrical charge and discharge of the surface states to values small enough to eliminate electrical drift problems. Surface states on a silicon crystal can be passivated by a silicon dioxide layer produced by thermal oxidation, for example. Surface states on gallium arsenide can be passivated by a layer of semiconducting material other than gallium arsenide, for example, providing the interface between the gallium arsenide surface and the semiconducting layer is appropriately controlled. The resulting junction between the semiconductor surface and the semiconducting layer is called a "heterojunction".
Passivation layers act as insulators and protect against electrical shorting and low breakdown voltages. Passivation layers also act as potting materials that protect against surface scratches and thus prevent electrical shorting.
During some processes of semiconductor device handling vacuum wands or other tools are used to move wafers or chips from one position to another. In such cases, the tool may cause a displacement or abrasion of exposed metal conductors that are formed on the wafers. Passivation layers help to eliminate this problem. Another problem that is encountered is found with semiconductor devices that incorporate an air bridge, which is a metal connection to a metal conductor that skips over an adjacent conductor, so that capacitive coupling is not added between the two conductors. The air disposed between the air bridge and the skipped over conductor has a low dielectric constant of nearly unity. However, if the metal air bridge is subjected to mechanical pressure causing it to contact the skipped over metal conductor, an electrical short would result. A passivation layer overlying the skipped over metal conductor can act as an electrical insulator to insure against such shorting.
Passivation layers generally are composed of silicon dioxide or silicon nitride, for example. Passivation layers using such materials are usually relatively thick, about 2000.ANG. or more, and require long deposition time, which adds to the cost of the semiconductor devices. With dielectric constants more than three times that of air these thick layers increase the capacitances between various parts of a semiconductor device, thereby degrading the device's high-frequency performance. Also, silicon dioxide and silicon nitride do not adhere very well to gold which is used for electrodes or conductors and actually are known to separate from gold conductors so that circuit problems are caused. Thick layers of insulators or passivation material using silicon dioxide or silicon nitride are subject to strain, and as they are relatively brittle in nature, can experience cracking and do not seal well. Furthermore, insulators such as silicon dioxide or silicon nitride do little to passivate surface states on some semiconductors such as gallium arsenide and can themselves act as charge traps and produce drift problems.